Antffiloom networks for use with dis



Sept. 10, 1963 ANTIBLOOM NETw'oRKs L B GANGAWERE OF THE CATHODE RAY TUBETYPE Filed Sept. 20, 1960 FOR USE WITH DISPLAY TUBES IN VEN TOR. AZ 0Y0B. GANG/[WERE A FOP/V5 Y5 United States Patent ANTInLooM NETwoiu-rs FORUSE WITH nrs- PLAY TUBES on THE CATHODE RAY TUBE TYPE Lloyd B.Gangawere, Baltimore, Md., assignor, by mesue This invention rel-ates toprotective circuits and more particularlyto antib'l-ocm networks forcathode ray tube (CRT) type circuits or storage tube type circuits toprotect these tubes from excessively high conduction during transientperiods of the application of high voltage to change the tube circuitfrom the standby condition to the operate condition.

In circuits employing cathode ray tubes for the display of videoinformation by intensity modulation of the electron stream, one problemmust be solved both in the interest of appearance and of long displaytube life. That problem is the transient state between the standbycondition where only filament and biasing voltages are applied to tubecircuits for maintaining these tubes in a standby condition and theoperate condition at which time all other operating voltages are appliedto the tubes; If the operating voltage is applied to the cathode of theelectron gun, a Very usual method for CRTs and the only practical method'for storage tubes at the present state-ofltheart, and the video signalsapplied to the control grid of this electron gun through a capacitor,the result is a very large current flow to the screen of the displaytube, limited only by the current capacity of the high voltage powersupply, until the video coupling capacitor is charged to the potentialof the cathode voltage through the display tube grid resistor andthrough the grid. This time may be as long as several seconds dependingon the size of the grid resistor and the video coupling capacitor. Theresults of this transient state produces an unpleasant flash of light onthe screen of the display tube or CRT known to those skilled in the artas blooming. This transient state may also cause ultimate, if notimmediate, destruction of the cathode in the CRT, and ultimate, if notimmediate, destruction of the storage properties of the storage grid inthe case where a storage tube is used. Another result is, that it isquite possible that the grid clamping diode may be destroyed because ofexcessive inverse voltage developed across the CRT resistor.

Several solutions are possible, one of which may be in grounding thecathode and in operating the second anodeat a positive potential. Thissolution requires that the deflection plates of the tube be operated atthe same high potential and therefore requires four high voltagecoupling capacitors for each electron gun and precludes the possibilityof direct coupling for the. display of very low frequency or directcurrent deflection signals. This solution is not practical for otherelectrical reasons for a storage tube. Another solution may be tocathodecouple the video signals to the CRT, leaving the grid free oflarge capacities. This method requires a low impedance source ofnegative going video signals. This may be \a satisfactory solution insome cases but has certain requirements which may not be desirable suchas high power requirements for the video output stage and a highcapacity, high voltage coupling capacitor, or a video outputtransformer, usually large and expensive. Still another solution may bein the use of a self-biased diode, of the vacuum tube or semiconductortype, across the grid and cathode of the CRT. This has a disadvantage ofrequiring a very high resistance (on the order of to 20 megohms) and avery large capacitor (at least ten times the value of the CRT gridcoupling capacitor). In the case of using a semiconductor diode, it mayincrease the input capacity of the CRT from 2 to 5 times its normalinput capacity depending on the choice of diodes and how high the backbiasing voltage becomes. For video display, this capacity may not betolerable.

Although there are probably other solutions, this in.- vention disclosespossible protectivenetwork of using a diode to protect a CRT or storagetube as well as the biasing circuitry and clamping diodes of such CRT orstorage tube circuits. The diode is coupled across the grid and cathodeof the tube to be protected through a portion of the biasing circuitryso that high current, which normally would pass through the-biasingcircuitry or cathode-(grid circuitry of the CRT to charge the videocoupling capacitor, would be bypassed through the diode protectivenetwork. In the diode the filament is energized through a separate andisolated filament circuit which energizes the diode filament in thestandby condition, but upon application of: high voltage to the CRT thisfilament circuit is broken. By this means the bypass conduction of highvoltage charging the video coupling capacitor exists only during thecooling period or the diode filament. This controlled bypass of theprotective network .preventsdamaging currents from passing through thebiasing circuitry or the. grid-tocathode circuitry of a CRT or storagetube during the transient period from standby to operate condition. Itis therefore a general object of this invention to provide protectivecircuitry to bypass high transient currents produced by high voltagesacross theelectrodes of a cathode ray tube or storage tube during thetransient periods from standby to operate conditions.

These and other objects and the attendant advantages, features, and usesmay become more apparent to those skilled in the art as this descriptionproceeds when considered along with the accompanying FIGURE of drawingillustrating one form of a protective nework using a diode.

Referring more particularly to the figure, there is illustrated for thepurpose of describing this invention a portion of a cathode ray tube 10with the grid 11 and cathode 12 shown therein. Although this inventionmay be equally applicable to storage tubes, the grid and cathodeelements of a cathode ray tube are illustrated herein for the purpose ofconvenience and to avoid any complication of .multiplying theembodiments to describe the invention. The grid 11 of the CRT 10 iscoupled to a video signal source at the terminal 13 through a videocoupling capacitor 14. The cathode 12 of CRT 10 is coupled to a commonconductor'15 to. which voltage maybe switched by means of the switchingmeans 16 having the switched positions of off, standby, and on, wherebyhigh voltage from a high voltage source 17 may be applied to the cathodeand other CRT circuitry. The grid 11 of CRT 10 is biased by a biasingresistor 18 and a portion of the resistance in a potentiometer 19through the adjustable tap 20 thereof. The resistance element ofpotentiometer 19 is coupled in series with a fixed resistance 21 acrossa biasing voltage 22. The biasing voltage 22 is switched on and offthrough a switch means 16b which is mechanically coupled to theprincipal switch 16. Clamping diode 23 is coupled in parallel with thebiasing resistance 18 to clamp voltages between the grid and cathode 11and 12 of the CRT 10. The junction of the clamping diode 23 and biasingresistance 18 is coupled through a capacitor 24 to the common conductor15. One end of the resistance element of the potentiometer 19 is coupledto the common conductor 15 and a voltage regulator tube 25 is coupled inparallel with the potentiometer 19 to provide a regulated supply of thebiasing voltage 22 to the grid 11 of the cathode ray tube and to theanode of the clamping diode 23. The above described circuitry is thatusually and normally found in general for display tube circuits such asCRTs. If switch 16 is switched to on from the standby position in whichbiasing potentials have been supplied to the cathode ray tube circuit, ahigh voltage would immediately be applied to the cathode 12 of CRT 10and to the biasing circuits by way of the common conductor means 15. Thevideo coupling capacitor 14 would immediately charge to the high voltageapplied plus the biasing voltage and in so doing high currents wouldhave to flow across the grid and cathode 11 and 12 of the CRT and alsothrough the grid biasing resistor 13 and a portion of the potentiometer19 to charge the capacitor 14-. This eventually, and sometimesimmediately, destroys the CRT or its biasing circuitry and sometimes theclamping diode 23. Accordingly, a protective network is most desirableto increase the reliability of such CRT or display tube circuits.

In the illustrated form of this invention a diode 30 is anode coupled tothe grid 11 of the CRT 10 and is cathode-coupled to the junction of thepotentiometer 19 and fixed resistance 21 of the biasing circuitry. Thefilament 31 of the diode 30 is coupled in circuit relation with thesecondary 32 of a filament transformer 33 the primary 34 of which may becoupled to an alternating current source through a switch 160. Theswitch Me is mechanically coupled to Work as a compound switch with theswitch components 16 and 16b, the standby position of which is coupledthrough the primary 34 from the alternating current source 35. The diode3% should be a high conductance diode and the filament transformer 33must have its secondary insulated from the high voltage used in thesystem. The filament voltage source 35 must be controlled independentlyof other filament voltages in the CRT system. For each additional CRTelectron gun in the system only an additional diode is required sinceall diodes can be operated from the same filament transformer. Aseparate bias voltage supply is shown for the circuit in the figurealthough it is to be understood that this bias may be derived from thehigh voltage supply if desired.

In the operation of the circuit shown in the figure let it be assumedthat switch 16 is thrown to the standby posit-ion at which time thefilament supply voltage 3-5 is supplied to the filament 31 of the diode30 by way of the filament transformer 33 and that all other filamentvoltages (not shown) are supplied to the CRT and related circuitry aswell understood by those skilled in the art. When it is desired to placethe CRT system into the operative position at which time it wouldprovide the display of targetsor other objects on the screen of the CRT(not shown), switch 16 should be thrown to the on position at which timehigh voltage from the source 17 is applied to the circuit. If a videosignal were applied at terminal 13 at this time, and there was noprotective network, CRT 10 would go to full conduction since the biasvoltage on CRT 10 would be zero. It is to be noted at this time thatswitch section 160 breaks the filament voltage circuit to the filament31 in the diode 30 and this filament starts to cool. The video couplingcapacitor 14 is already charged to the bias supply voltage and nowcharges to the high voltage potential of 17 by way of the cathode andanode of the diode 30 which diode 30 will at the same time hold aconstant difference of potential between the grid 11 and cathode 12 ofthe CRT 10 keeping it cut ofi. This difference of potential is verynearly equal to the bias voltage since the drop between the anode andcathode of the diode 30 is very small. The time required for chargingthe video coupling capacitor 14 will vary with circuit values and thecurrent capabilities of the high voltage power supply 17 but this timewill be on the order of one millisecond. When the filament 31 of thediode 3b is sufficiently cool, conduction will cease and this diode willbe cut ofi so that normal circuit voltage conditions are restoredthrough the action of the clamping diode 23 whereby normal CRT 19operation may be maintained for the purpose intended. The CRT 10 and itsrelated biasing circuit and clamping diode circuit have thereby beenprotected from high transient currents which would normally flowtherethrough to charge the video coupling capacitor 14. The anode andcathode of the protective diode 30 obviously will produce some capacityin the CRT circuit which may vary from 2 to 10 rnicro-microfaradsdepending on the diode chosen. This value represents from 5 to 10percent of the normal capacitance of the CRT circuit and is not serious.The time to recover normal circuit functions will vary depending on thethermal inertia of the cathode of tube 30. In most cases, this recoverytime can be tolerated in view of its protective characteristics.

While many modifications and changes may be made in the constructionaldetails and features of this invention without departing from the spiritand scope of this invention, it is to be understood that I desire to belimited in my invention only by the scope of the appended claims.

I claim:

'1. An antibloom network for use with standby radar recording displaytube circuits having a cathode and a grid with the cathode in a switchcircuit to high voltage to switch said recording display tube from astandby condition to an operative recording display condition and withthe grid coupled to a biasing voltage and branch coupled through acoupling capacitor to a signal source, the invention which comprises:

a diode having an anode coupled to the grid of said recording displaytube, having a cathode coupled through a resistance to the cathode ofsaid recording display tube to provide a conduction path parallel tosaid recording display tube grid and cathode, and having a filament; and

filament control means coupled to said filament and to a filament supplyvoltage for controlling the supply of voltage to said filament andthereby conduction of said diode a predetermined interval of time afterswitching high voltage to said recording display tube cathode andthereafter control said diode to nonconduction whereby blooming of saiddisplay tube is prevented by virtue of bypassing high currents aroundsaid recording display tube cathode and grid through said diode to saidcoupling capacitor for said predetermined interval of time until saidcoupling capacitor is fully charged to said high voltage.

2. An antibloom network as set forth in claim 1 wherein said filamentcontrol means is a transformer having the secondary therof providingsaid coupling to said diode filament and a primary switchable in saidcoupling to said filament supply voltage with the switch thereofactuable in alternate closing and opening sequence with the high voltageopening and closing sequence, respectively, whereby said predeterminedinterval of time of diode conduction is determinative from filamentcooling.

References Cited in the file of this patent UNITED STATES PATENTS2,457,112 Abercrombie et al Dec. 28, 1948 2,756,378 Scott July 24, 1956FOREIGN PATENTS 1,055,699 Germany Apr. 23, 1959

1. AN ANTIBLOOM NETWORK FOR USE WITH STANDBY RADAR RECORDING DISPLAYTUBE CIRCUITS HAVING A CATHODE AND A GRID WITH THE CATHODE IN A SWITCHCIRCUIT TO HIGH VOLTAGE TO SWITCH SAID RECORDING DISPLAY TUBE FROM ASTANDBY CONDITION TO AN OPERATIVE RECORDING DISPLAY CONDITION AND WITHTHE GRID COUPLED TO A BIASING VOLTAGE AND BRANCH COUPLED THROUGH ACOUPLING CAPACITOR TO A SIGNAL SOURCE, THE INVENTION WHICH COMPRISES: ADIODE HAVING AN ANODE COUPLED TO THE GRID OF SAID RECORDING DISPLAYTUBE, HAVING A CATHODE COUPLED THROUGH A RESISTANCE TO THE CATHODE OFSAID RECORDING DISPLAY TUBE TO PROVIDE A CONDUCTION PATH PARALLEL TOSAID RECORDING DISPLAY TUBE GRID AND CATHODE, AND HAVING A FILAMENT; ANDFILAMENT CONTROL MEANS COUPLED TO SAID FILAMENT AND TO A FILAMENT SUPPLYVOLTAGE FOR CONTROLLING THE SUPPLY OF VOLTAGE TO SAID FILAMENT ANDTHEREBY CONDUCTION OF SAID DIODE A PREDETERMINED INTERVAL OF TIME AFTERSWITCHING HIGH VOLTAGE TO SAID RECORDING DISPLAY TUBE CATHODE ANDTHEREAFTER CONTROL SAID DIODE TO NONCONDUCTION WHEREBY BLOOMING OF SAIDDISPLAY TUBE IS PREVENTED BY VIRTUE OF BYPASSING HIGH CURRENTS AROUNDSAID RECORDING DISPLAY TUBE CATHODE AND GRID THROUGH SAID DIODE TO SAIDCOUPLING CAPACITOR FOR SAID PREDETERMINED INTERVAL OF TIME UNTIL SAIDCOUPLING CAPACITOR IS FULLY CHARGED TO SAID HIGH VOLTAGE.